1. Field of the Invention
The present invention is directed generally to micro-electromechanical systems and devices (MEMS devices), and more particularly to MEMS devices having an integral electrical isolation structure.
2. Background Art
MEMS devices are electrical and mechanical devices that are fabricated at substantially microscopic dimensions utilizing techniques well known in the manufacture of integrated circuits. Present commercial applications of MEMS technology are predominantly in pressure and inertial sensing, for example, accelerometers and gyroscopes used in hand-held devices, for example, wireless telephones. One common implementation of a MEMS device used to detect acceleration is a capacitive sensor that utilizes an array of cantilever beams, for example, MEMS devices fabricated as described in U.S. Pat. Nos. 6,342,430 and 6,239,473, both of which are incorporated by reference herein.
FIG. 1 schematically illustrates a beam 200 fabricated according to the methods described in U.S. Pat. Nos. 6,342,430 and 6,239,473. The beam 200 has an isolation joint 300 between a distal portion 220 of and a proximal portion 240 of the beam. The terms distal and proximal indicate position relative to a connection with a substrate (not shown) from which the beam is suspended. The isolation joint 300 breaks the electrical connection between the distal portion 220 and the proximal portion 240 of the beam 200. The proximal portion 240 of the beam can be directly or indirectly connected to a substrate (not shown). For example, beam 200 can be cantilevered over a surface of the substrate, which allows the beam 200 to move or flex. When viewed from above, the isolation joint 300 is linear.
FIG. 2 is a cross-sectional view of the isolation joint 300 of FIG. 1 taken along line 2-2. The isolation joint 300 can be formed by etching a linear trench in a substrate, for example, a silicon wafer, and subsequently oxidizing the substrate to create an oxide, for example, silicon dioxide. The oxidation process consumes the silicon sidewalls of the trench to form silicon dioxide, and the sidewalls encroach upon each other as they are converted to silicon dioxide. Typically, the sidewalls contact each other at seam 320 an interface that is not chemically bonded. The side profile of the trench can be reentrant or wider at the bottom than at the top. During the manufacturing process, a reentrant profile can help prevent the formation of silicon stringers that run between the distal portion 220 and the proximal portion 240 of the beam 200, which would prevent effective electrical isolation. Because a reentrant profile is narrower at the top than at the bottom, the top of the isolation trench closes at seam 320 before the bottom of the isolation trench is completely oxidized. Closing the top can cause a void 340 to be formed in a lower half of the isolation joint 300.
MEMS devices having a beam microstructure as described above operate in varied environmental conditions. For example, MEMS devices can be subject to temperature variations, humidity exposure, and mechanical shocks. In addition to being electrically isolated between the beam and the substrate, each beam must have adequate mechanical strength to not fail when subjected to a mechanical shock to function properly. However, the seam 320 and void 340 compromises the mechanical properties of the isolation joint 300 and the beam 200 as a whole. The seam 320 and void 340 allows the entire beam to flex or hinge when a force is applied to the beam 200. A force created by a shock event can cause the beam 200 to deflect, and the resulting strain can lead to a fracture of the beam 200 and failure of the MEMS device.
Accordingly, there is need for improved MEMS devices that can better withstand mechanical shocks.